Varying damping arrangements for wind turbines have been suggested in the patent literature.
For example, U.S. Pat. No. 6,672,837 relates to a wind turbine with oscillation damping means provided at the nacelle. The oscillation damping means is designed for damping edgewise oscillations of the rotor blades in the rotational plane of the rotor.
In particular, the oscillation damping means suggested in U.S. Pat. No. 6,672,837 relates to a wind turbine in which the oscillation damping means are provided at the end of the nacelle, i.e. opposite the end from which the rotor extends. The damping means are designed for damping oscillations of the first fundamental frequency of the rotor blades in the rotational plane, especially oscillations being perpendicular to the rotational axis of the rotor.
Also, various electrically based damping arrangements have been suggested. For example, US 2006/066111 discloses a vibration damping technique for a wind turbine system. The wind turbine system includes a vibration damper which provides a variable signal to control torque produced by a generator of the wind turbine system. The variable signal is based on generator speed and has a first local peak value based on a resonance frequency of tower side-to-side oscillation. Another electrically based damping arrangement is suggested in EP 1 045 988.
It may be seen as an object of embodiments of the present invention to provide a method and an arrangement for damping rotor blade induced oscillations in wind turbines.
It may be seen as a further object of embodiments of the present invention to provide a method and an arrangement which can easily be installed in existing wind turbines. Other objects appear from the description and figures.